SYDNEY (Reuters) - Australia’s military intercepted a Sri Lankan boat carrying 13 asylum-seekers last month, Minister for Home Affairs Peter Dutton said on Monday, as Canberra sought to defend the relocation of a Tamil family to a remote detention centre.

Dutton said the vessel - intercepted off the Cocos Islands, a remote Australian territory in the Indian Ocean - was the 13th boat from Sri Lanka attempting to travel to Australia to seek asylum in the past 18 months.

“It is the reason Sri Lanka was the first country I visited after the election, to make sure we can keep these boats stopped,” Dutton told the Courier-Mail newspaper. Australia’s conservative government was re-elected in May.

“This threat is very real,” he said.

Under Canberra’s hardline immigration policy, would-be asylum-seekers intercepted at sea while trying to reach Australia are returned to the boat’s country of origin.

Asylum-seekers who reach Australia are sent to Australian-run detention camps in Papua New Guinea and the South Pacific island of Nauru, where they are held in conditions widely criticised by organisations such as the United Nations.

However, critics accuse the Australian government of seeking to sway public opinion following the relocation of a Tamil asylum-seeker family to Christmas Island on Saturday.

“The government would not talk about it but when it is convenient, you get this,” Joel Fitzgibbon, an opposition Labor lawmaker, told Channel 9 television.

Thousands of people across Australia protested at the weekend against the decision but Prime Minister Scott Morrison said on Monday allowing the Tamil family to stay would encourage further attempts by asylum-seekers.

A detention centre on Christmas Island - some 1,550 km (960 miles) northwest of the mainland - was reopened earlier this year after it lay idle for 10 years.

Known only by their first names, Priya and Nadesalingam arrived in Australia separately as asylum-seekers in 2012 and 2013, before Canberra began holding asylum-seekers offshore.

The couple had two daughters, now aged 4 and 2, after marrying and settling in a small town in Australia’s northeast.

Despite the protests, Dutton said the family will not be allowed to stay after their applications for asylum were refused.